The present invention relates to a method for high speed testing of whether a metallic cover has a leak by the application of pressurized air, wherein said metallic cover has an opening-defining score line on the upper surface and a tab for breaking off a part of the cover defined by the score line and which is fixed to the upper surface side of the cover, and said cover further comprises a central plate portion, a countersink portion, a flange portion, and a curled portion positioned from the center of the cover to its periphery in the aforesaid order.
The above-mentioned type metallic cover is likely to suffer from breakage at the tab-fixing portion and along the score line when it is manufactured. Therefore, hitherto tests of whether such breakage is present or not have been widely performed in the can manufacturing industry by the utilization of pressurized air. The prior art method for the test comprises holding a metallic cover between the upper and lower parts facing each other to form a closed chamber which is divided into two closed portions wherein said metallic cover is the boundary, injecting pressurized air into the closed portion which faces to the tab-absent surface or the lower surface of the cover and to which the inner pressure is applied when the cover is seamed to a can, such as a beer can, containing a liquid which generates an inner pressure, and testing whether said metallic cover has a leak with a sensor which functions in response to the change in the pressure within the other closed portion which faces the tab-existing surface or the upper surface of the cover caused by the passage of the pressurized air into it through said leak and gives a signal warning that said metallic cover has a leak.
The main system of an example of a prior test machine employed in the above type method is shown in FIG. 1.
FIG. 1 is a sectional view of the main system for testing whether a metallic cover has a leak in the state where the metallic cover is inserted between the upper and lower parts in a closed chamber.
In this figure, A is the upper part and B is the lower part; the two parts construct a closed chamber.
The upper part A comprises a ring-like wall 1, a cylindrical wall 2 hanging down from the periphery of said ring-like wall 1 and a knockout part 3 which can move up and down through the center of said ring-like wall 1 and is always pushed downwardly by a spring 5 located upwards. Here, the lower portion 4 of said knockout part is exposed to space 6 surrounded by the above-noted cylindrical wall 2. Furthermore, there is provided a passage 7 in said knockout part 3 which communicates with a pipe 8 leading to a detector which detects the change in the air pressure (not shown) and is open to said space 6 on the other side. In order to form one closed chamber together with the lower part B, an outer elastic ring 10 is attached to the lower surface 9 of the above cylindrical wall 2, and an inner elastic ring 12 is provided near the periphery of the lower surface 11 of the ring-like wall 1 which contacts with the upper surface of the flange portion of the metallic cover so that said closed chamber is divided into two upper and lower closed portions when the metallic cover C is placed between the upper and the lower parts A and B.
The lower part B comprises a disc wall 21 and an annular projecting wall 23 which projects upwards from the periphery of the upper surface 22 of said disc wall 21 at position at which the lower part contacts the above inner elastic ring 12; the periphery of the disc wall 21 forms a ring portion 24 outside said annular projecting wall 23 to receive the outer elastic ring 10. In the center of said disc wall 21 there is provided a passage 25 which communicates with a pressurized air supply pipe 28 and is open in the upper surface 22 of the disc wall 21. Furthermore, the upper edge surface of the projecting wall 23 is provided with a staged notch 26 which receives the edge portion of the curled portion 33 (described later) of the cover C.
The metallic cover shown in FIG. 1 comprises a central plate portion 30, a countersink portion 31, a flange portion 32, the curled portion 33 and a pull tab 35 which is fixed to the upper surface side of said central plate portion 30 by a rivet 34 formed in the center of the central plate portion 30, and a score (not shown) defining an opening is coined in the central plate portion 30.
Hereunder, how the prior art for testing whether a cover has a leak is carried out in the thus formed closed chamber is described. First, the cover C is put on the lower part B by placing the curled portion 33 on the staged notch 26 of the annular projecting wall 23 of the lower part B. Then, the upper part A and the lower part B are positioned close to each other so that the annular projecting wall 23 of the lower part B comes in the inside of the cylindrical wall 2 of the upper part A, and the outer elastic ring 10 comes in contact with the ring portion 24 of the lower part B while the upper surface of the flange portion 32 of the metallic cover is contacted with the inner elastic ring 12 of the upper part A under pressure, thereby holding the metallic cover C between the upper part A and the lower part B.
That is, in the above system, the upper part and the lower part get positioned close to each other to contact the outer elastic ring 10 of the upper part with the ring portion 24 of the lower part under pressure, thereby forming the closed chamber and positioning the peripheral portion (the flange portion and the curled portion) of the metallic cover between the inner elastic ring 12 and the staged notch 26 in the upper edge of the annular projecting wall 23 of the lower part under pressure to divide the closed chamber into an upper closed portion 36 and a lower closed portion 37 making the metallic cover C the boundary (FIG. 1). Either of the two closed portions 36 and 37 has a considerable large space around the central plate portion of the metallic cover.
After the cover is put into the state as shown in FIG. 1, pressurized air is supplied to the inside of the lower closed portion 37 through the pressurized air supply pipe 28 and the passage 25. Thus, if the metallic cover has a leak portion such as a pinhole or a crack along the score line, the pressurized air in the lower closed portion 37 flows into the upper closed portion 36 through said leak portion resulting in the change in the air pressure within said upper closed portion 36, and said change is detected with a detector via the passage 7 and the pipe 8 of the knockout part to generate a signal.
In case the metallic cover has no leak portion anywhere, the air pressure within the upper closed portion 36 does not change, and thus the detector does not generate a signal.
In connection with the above, the knockout part 3 mentioned above is to prevent the tested cover from being left in the upper part and to keep it in the original position on the lower part when the upper and the lower parts are suddenly separated from each other. That is, when the upper part moves up, the knockout part 3 moves down to hold back the cover so that it does not lift together with the upper part.
Thus, in a method where pressurized air is injected into one of the two closed portions formed by the division of the closed chamber making the metallic cover the boundary, wherein the determination of whether the cover has a leak portion is tested by the change in the air pressure within the other closed portion because of the passage of the pressurized air thereinto through the leak portion if the metallic cover has a leak portion such as a crack or a pinhole, it is desirable to improve the detection capacity of the detector to such an extent that it can detect a very small change in air pressure in order to shorten the test time or to effect the test at a high speed. Alternatively, air at a higher pressure is injected. In this case, the detection ability of the test machine may be the same level as of the prior art one. That is, when the size of the leak portion is the same, it is expected that the higher the pressure of the air injected is, the shorter the time may become which is necessary for the air to flow in such an increased amount that the detector can detect its flow into the other closed portion. This means that the air pressure within "the other closed portion" is increased more speedily, and thus the air pressure attains the minimum level which the detector can detect (this level is definite) more speedily, thereby shortening the test time. In this connection, however, it is noted that the countersink portion of the cover may be deformed or buckled by the application of the increased pressurized air to the central portion because the periphery of the cover is fixed.
Now, having the above-mentioned expectation, the inventors of the present invention tried some experiments using the prior art system referred to above.
As a result, it was found that covers may be tested at a rate speed of about 300 pieces/min. in case the pressure of the air injected is 0.7 kg/cm.sup.2 and that the detectable minimum diameter of the leak aperture is about 3/1000 inches. Here, no occurrence of deformed covers was found.
Then, the air pressure was gradually increased, but the test speed did not increase correspondingly while the occurrence of deformed covers was noted.
For example, when the air pressure was set at 3 kg/cm.sup.2, the treatment rate attained about 350 pieces/min., but some of the covers were deformed to such an extent that they could not be used. That is, it was found that, as the central plate portion was moved upwards by the application of the pressurized air thereby deforming the countersink portion, as a means to prevent this, the knockout part was utilized to contact and push down the cover, but, what is directly received by the knockout part is the pull tab, and, as there is a gap of a size corresponding to the thickness of said tab between the central plate portion which cannot be received directly by the knockout portion and the latter, a recess in the form corresponding to that of the tab is produced in the central plate portion thereby producing a negative result in that said deformation causes breakage of a part of the score line or weakens the line even if the line is not broken.
From this fact, we, the inventors of the present invention, knew that there is a limit to the shortening of the test time by the application of air injected at an increased pressure with the prior art system, and thus a satisfactory result cannot be expected by the application of the prior art. Instead, we have studied other ways to produce a satisfactory result. As a result, we have attained the present invention.